1. Field of the Invention
The present invention relates to a secondary cell, and more particularly to a spinel-type LiMn secondary cell.
2. Description of the Related Art
Electric vehicles and hybrid cars are presently developed as motor-driven mobile vehicles primarily for the purpose of environmental protection, and there is a demand for small, lightweight, high-performance secondary batteries as a power supply for those vehicles.
Such secondary batteries include a spinel-type LiMn secondary cell as disclosed in Japanese Patent Laid-open Publication No. 92250/1997 and Japanese Patent Laid-open Publication No. 133221/2000. The spinel-type LiMn secondary cell is small and lightweight, has a large capacity, and provides good charging characteristics and cyclic characteristics.
One conventional spinel-type LiMn secondary cell will be described below with reference to FIG. 1 of the accompanying drawings. As shown in FIG. 1, the conventional spinel-type LiMn secondary cell, denoted by 100, has cell casing 101 and electrode unit 102 housed in cell casing 101.
Cell casing 101 comprises main member 103 and lid member 104, each made of iron placed with nickel. Main member 103 is in the form of a hollow cylinder having a closed lower end and an open upper end. Lid member 104 is in the form of a disk closing the open upper end of main member 103.
Lid member 104 has circular through hole 105 defined centrally therein, and is welded to the open upper end of main member 103. Electrode unit 102 comprises a positive electrode sheet, a negative electrode sheet (both not shown), and hollow core 106. The positive electrode sheet and the negative electrode sheet, which are laminated together with a separator sheet interposed there between, are wound into a cylindrical column around core 106.
The positive electrode sheet has its surfaces uniformly coated with a powdery positive electrode active material (not shown), and the negative electrode sheet has its surfaces uniformly coated with a powdery negative electrode active material. The surfaces of the positive and negative electrode sheets refer to both face and back sides thereof.
The positive electrode sheet is made of pure aluminum, and the negative electrode sheet is made of pure copper. The positive electrode active material comprises a compound including lithium and manganese as indispensable constituents, and may, for example, be a powder of LiMn2O4. The negative electrode active material comprises a compound including carbon as an indispensable constituent. The gap between the sheets of electrode unit 102 is impregnated with a nonaqueous electrolytic solution.
A plurality of positive electrode tabs 107 made of pure aluminum project upwardly from an upper edge of the positive electrode sheet, and a plurality of negative electrode tabs 108 made of pure copper project downwardly from a lower edge of the negative electrode sheet. Therefore, positive electrode tabs 107 project upwardly from respective positions on the upper surface of electrode unit 102, and negative electrode tabs 108 project downwardly from respective positions on the lower surface of electrode unit 102.
Negative electrode tabs 108 projecting downwardly from the lower surface of electrode unit 102 are bent toward the center of electrode unit 102, and superposed at the center of electrode unit 102 and welded directly to the inner surface of the bottom of main member 103. Positive electrode tabs 107 projecting upwardly from the upper surface of electrode unit 102 are welded to the bottom of positive electrode member 110.
Positive electrode member 110 is mounted in circular through hole 105 in lid member 104 by insulating members 111, 112.
Insulating members 111, 112 are made of polypropylene, and are in the from of a pair of annular members held in intimate contact with lower and upper surfaces of a flange of lid member 104 that defines through hole 105 centrally in lid member 104.
Positive electrode member 110 comprises knurled bolt 113 and knurled nut 114, each made of pure aluminum. Knurled bolt 113 extends upwardly in through hole 105 in lid member 104 with insulating member 111 interposed between knurled bolt 113 and lid member 104, and knurled nut 114 is threaded on knurled bolt 113 with insulating member 112 interposed between insulating member 112 and lid member 104, thus hermetically closing cell casing 101. Positive electrode tabs 107 are welded to the bottom of the head of knurled bolt 113 that is positioned within cell casing 101.
Since the positive electrode sheet and the negative electrode sheet, between which the nonaqueous electrolytic solution is impregnated, develop positive and negative potentials, respectively, the threaded stud of knurled bolt 113 which projects upwardly from the upper surface of LiMn secondary cell 100 functions as a positive electrode, and the lower surface of main member 103 functions as a negative electrode.
LiMn secondary cell 100 is of a large size as a whole for use on a motor-driven mobile vehicle such as an electric vehicle. Lid member 104 of cell casing 101 is welded to main member 103 thereof for giving LiMn secondary cell 100 a desired level of mechanical strength. Therefore, main member 103 cannot be insulated from lid member 104, which thus cannot be used as a positive electrode.
Instead, positive electrode member 110 mounted on lid member 104 by insulating members 111, 112 functions as a positive electrode that is insulated from cell casing 101.
Though positive electrode member 110, positive electrode tabs 107, and the positive electrode sheet are connected to each other, they are made of pure aluminum and are not subject to an unwanted chemical reaction such as electrolytic corrosion.
Since positive electrode member 110 comprises knurled bolt 113 and knurled nut 114, which are generally available parts, positive electrode member 110 is simple in structure, can be constructed from an existing product, and can easily be installed on cell casing 101.
As the threaded stud of knurled bolt 113 which functions as a positive electrode projects from cell casing 101, it may be engaged by a positive electrode terminal of a motor-driven mobile vehicle, which may be tightened in place by a hexagonal nut (not shown) threaded over the threaded stud of knurled bolt 113.
As described above, because lid member 104 is welded to main member 103 as a negative electrode to achieve a desired level of mechanical strength, positive electrode member 110 as a positive electrode is mounted in through hole 105 by insulating members 111, 112, and is made of pure aluminum as with the positive electrode sheet and positive electrode tabs 107 in order to prevent an unwanted chemical reaction.
However, inasmuch as positive electrode member 110 comprises knurled body 113 and knurled nut 114, it suffers a lack of mechanical strength as it is made of pure aluminum. Particularly, while a positive electrode terminal can easily be attached to and removed from knurled bolt 113 by a hexagonal nut, it is highly likely for the threads of the knurled bolt 113 to be worn out when a positive electrode terminal is repeatedly attached to and removed from knurled bolt 113.
Generally, a hexagonal nut for tightening a positive electrode terminal on knurled bolt 113 is made of iron, but not aluminum, for a reduced cost. Repeated tightening of an iron hexagonal nut on knurled bolt 113 accelerates wear on knurled bolt 113.
When LiMn secondary cell 100 is installed on a motor-driven mobile vehicle, since LiMn secondary cell 100 is subject to frequent vibrations and stresses, knurled bolt 113 tends to be worn at an accelerated rate.
In order to solve the above problems, JP92250/1997 discloses an attempt to increase the diameter of the knurled bolt which serves as a positive electrode and also increase the size of the threads of the knurled bolt. However, LiMn secondary cell 100 is not a product for independent use, but is mounted on a certain motor-driven apparatus, and hence is generally constructed according to various standards.
According to some standards, LiMn secondary cell 100 may possibly be constructed in a small size which makes it difficult to increase the diameter of positive electrode member 110. Though a positive electrode terminal can easily be attached to and removed from knurled bolt 113 by a hexagonal nut, as described above, the diameter of knurled bolt 113 and the size of the threads thereof cannot be increased in size if the positive electrode terminal and the hexagonal nut are standardized.
Positive electrode tabs 107 projecting from respective positions on the upper surface of electrode unit 102 are welded to the head of knurled bolt 113. If the diameter of knurled bolt 113 is increased, then positive electrode tabs 107 can be welded only to the bottom of the head of knurled bolt 113.
In the welding process, positive electrode tabs 107 that project axially of knurled bolt 113 need to be bent substantially at a right angle and welded to the bottom of the head of knurled bolt 113. At this time, as shown in FIG. 2 of the accompanying drawings, since electrode unit 102 positioned below the bottom of the head of knurled bolt 113 interferes with welding machine 120, the welding process is low in efficiency, and the productivity of LiMn secondary cell 100 is low.
For easily welding positive electrode tabs 107 to the bottom of the head of knurled bolt 113, it is necessary to extend positive electrode tabs 107. However, as shown in FIG. 1, because extended positive electrode tabs 107 need to be bent in multiple layers and positioned between the upper surface of electrode unit 102 and the lower surface of knurled bolt 113, the gap between the upper surface of electrode unit 102 and the lower surface of knurled bolt 113 needs to be increased, resulting in an increased dead space in cell casing 101.
LiMn secondary cell 100 also suffer the above shortcomings if knurled bolt 113 is constructed as a hexagonal bolt, an Allen bolt, a Huck bolt, a rivet, or the like.
It is an object of the present invention to provide a LiMn secondary cell having a structure in which a positive electrode member is mounted in a through hole in a cell casing by an insulating assembly, and which is capable of increasing the mechanical strength of the positive electrode member without causing an unwanted chemical reaction.
A spinel-type LiMn secondary cell according to the present invention comprises an electrode unit, a conductive cell casing, and a conductive positive electrode member as major components. The electrode unit is housed in the cell casing, and the positive electrode member is mounted in the cell casing.
The electrode unit has a positive electrode sheet and a negative electrode sheet which are laminated together with a separator sheet interposed there between, and wound into a cylindrical column. The electrode unit is impregnated with a nonaqueous electrolytic solution between the sheets. The positive electrode sheet is coated on surfaces thereof with a powdery positive electrode active material and connected to the positive electrode member by positive electrode tabs. The negative electrode sheet is coated on surfaces thereof with a powdery negative electrode active material and connected to the cell casing by negative electrode tabs. Since the positive electrode member is mounted in a through hole in the cell casing by an insulating assembly, the conductive cell casing serves as a negative electrode, and the conductive positive electrode member as a positive electrode.
As is the case with the conventional LiMn secondary cell, the positive electrode sheet is made mainly of aluminum, and the positive electrode active material includes lithium and manganese as indispensable constituents. Unlike the conventional LiMn secondary cell, the conductive positive electrode member is made of an aluminum alloy with manganese mixed therewith.
Since the aluminum alloy with manganese mixed therewith is of better mechanical strength than pure aluminum, the positive electrode member of the LiMn secondary cell according to the present invention has good mechanical strength. Because manganese mixed with the aluminum alloy of the positive electrode member is an indispensable constituent of the positive electrode active material, it does not cause an unwanted chemical reaction such as electrolytic corrosion.
The mechanical strength referred to above means various mechanically required aspects of strength including hardness, tenacity, wear resistance, etc.
The conductive positive electrode member is made of a 3000 series aluminum alloy. Thus, the conductive positive electrode member can be made of an existing aluminum alloy to provide good mechanical strength and make itself free of an unwanted chemical reaction. The LiMn secondary cell can thus be manufactured with increased productivity.
The conductive positive electrode member comprises a bolt and a nut. Therefore, it is simple in structure and can be constructed from an existing product, making it possible to manufacture the LiMn secondary cell with increased productivity.
The bolt extends through the through hole out of the cell casing, and the nut is threaded on the bolt which projects out of the cell casing. With this arrangement, the positive electrode member can simply be mounted in the cell casing, and it is easy to have a separate positive electrode terminal engage the bolt of the positive electrode member that projects from the cell casing and tightened on the bolt by a hexagonal nut.
The positive electrode tabs are welded to an outer side surface of a head of the bolt. Since the positive electrode tabs can be welded to an outer side surfaces of the head of the bolt which does not need to be unduly large in diameter, without being largely bent, the LiMn secondary cell can thus be manufactured with increased productivity, and a dead space where the positive electrode tabs are positioned can be reduced.
The insulating assembly comprises a closing member closing a gap between the cell casing and the positive electrode member, and a retaining member keeping the cell casing and the positive electrode member positioned relatively to each other. The positive electrode member having sufficient mechanical strength to allow external stresses applied thereto to act on the joint between the positive electrode member and the cell casing. However, because the bonding strength and closure of the cell casing and the positive electrode member are provided by the retaining member and the closing member of the insulating assembly, the insulating assembly is prevented from being broken, and no short circuit occurs between the positive electrode member and the cell casing.
The cell casing comprises a cylindrical main member and a disk-shaped lid member, the lid member having the through hole defined therein, the main member having an open end, the lid member being welded to the open end of the main member. The electrode unit can easily be housed in the cell casing, and the cell casing is strong in its entirety with the through hole defined in one end thereof. The strength of exposed parts of the LiMn secondary cell can be increased as a whole.
The LiMn secondary cell can be manufactured by forming the positive electrode sheet mainly of aluminum, producing the positive electrode active material of lithium and manganese as indispensable constituents, and forming the positive electrode member of an aluminum alloy with manganese mixed therewith.
The method further comprises the step of ultrasonically welding the positive electrode tabs to the positive electrode member. A highly insulating oxide film is formed of its own accord on the surface of the positive electrode member that is made of an aluminum alloy with manganese mixed therewith. When the positive electrode tabs are ultrasonically welded to the positive electrode member, the oxide film is broken, allowing the positive electrode tabs to be well electrically connected to the positive electrode member.
A motor-driven mobile vehicle according to the present invention has a negative electrode terminal held against and electrically connected to at least the cell casing of the spinel-type LiMn secondary cell, a positive electrode terminal engaging and electrically connected to the bolt of the LiMn secondary cell by a nut. An electric motor on the motor-driven mobile vehicle is energizable by electric energy supplied from the negative electrode terminal and the positive electrode terminal, and a vehicle body supporting the electric motor and the spinel-type LiMn secondary cell is moved by a moving means with power produced by the electric motor.
The motor-driven mobile vehicle can be operated with the spinel-type LiMn secondary cell used as a power supply. Even when frequent vibrations and stresses are applied from the positive electrode terminal to the positive electrode member of the spinel-type LiMn secondary cell while the motor-driven mobile vehicle is moving, since the positive electrode member has good mechanical strength, it is prevented from being broken or unduly worn. The motor-driven mobile vehicle is thus of increased reliability and durability.
The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate an example of the present invention.